Liquids, such as products of the food processing industry are generally stored in glasses and bottles. Useful life of such glasses depends considerably on the extent of any damaging stresses in the glass. Stress-freedom of glasses filled under pressure is of special importance, to minimize the risk of explosion. Production technology of glassware such as bottles, by pressing or blowing, often results in damaging mechanical stresses. Detection and control of mechanical stresses in the bottom of the glassware represent a most important task both for reducing waste and obtaining maximum operative safety. Accordingly, a testing-qualifying method and apparatus therefor are needed, which is suitable for the final qualitative control of the glasses and thus it is imperative to develop a method for quick measurement and quality control
The aim of the invention is to perform quality control of the stresses raised in glass bottles, etc. on the basis of objective criteria and with a high sensitivity, by detecting the index of birefringence at the bottom of hollow glassware.
Presently this task is solved by a conventional polariscope, stress-testing apparatus, in the glass industry.
Function of the apparatus is based on the phenomenon of the tension-caused birefringence. Under the effect of mechanical stresses the glass, which is originally optically isotropic, becomes double-refractive or birefringent. That means that distribution of the refractive index is not spherical but ellipsoidal. State of polarization of the light passing through the glass which became birefringent, changes in comparison to the state of polarization of incident light, that means that generally the linearly polarized light will be converted into elliptically polarized light. Thus the glass to be tested is illuminated with linearly polarized white light from the bottom of the glass, through partly a light-diffuser screen permeable to light is used for homogenizing the cross-sectional intensity distribution of the illuminating light beam, followed by a polar filter with a large diameter /30 to 40 cm/. The glass is placed into the path of the light thus produced and the axis of the rotation of the glass is parallel with the illuminating light bundle. The light passing through the bottom of the glass bottle is detected visually through the second polar filter (analyzer) so that by rotating the axis of the glass object visible parts of the bottom can be examined through the mouth of the glass bottle. Colored strips (isochromates) become visible after the analyzer. These are characteristic of the stress distribution in the bottom of the bottle.
This prior art is described in greater detail e.g. by Miklos Vermes: Polar light, Technical Publishers, 1967; Manual of Glass Industry, Technical Publishers, Hungary 1964. Many methods are known for testing the mechanical stresses in optical materials. In one of these monochromatic planar polar incident light is used instead of white light, while a system of black strips is formed after the analyzer. This characterizes the planar distribution of the stresses. That test is performed in a most complicated fashion by qualified visual observation and analysis.
Summing up all known methods it can be stated, that:
1./ Detection is subjective, in particular the detection of low stresses, where a little color displacement increases uncertainty in measuring.
2./ Evaluation is slow, and there is no way to display the measured stress distribution. PA1 3./ These known methods are not suitable for producing a qualifying "measuring index" integrally characterizing the whole area of the glass bottom. PA1 4./ One cannot examine the entire area of the bottom at the same time by means of the usual parallel or diffuse illuminating methods in the case of bottles with a small neck diameter. PA1 5./ At low stresses the detectable light intensity is extraordinarily small, and the smallest detectable stress changes in dependence of the extent of polarization, monochromacity of the light source, and the ambient light. PA1 The bottoms of the glass bottles are to be illuminated from the mouth of the bottle with a conically shaped light bundle having a predetermined aperture angle to achieve simultaneous illumination of the whole bottom and to avoid the illumination of the sidewall of the bottle. PA1 Laser is used as a light source. The bundle of this source is collimated for producing light cones which are fitted to bottles with different heights. PA1 Linearly polarized laser beam is used, followed immediately by a polarizing filter, the transmission direction of which is parallel with the polarization plane of the laser bundle to reduce apolar or partly polar noise-light coming from the discharge tube of the laser, or from the ambiance, and to increase sensitivity. PA1 Cross-sectional intensity distribution of the Gaussian-distribution of the laser bundle is so influenced by the optical system that the light cone should uniformly illuminate the whole area of the bottom with a homogeneous intensity to assure simultaneous integrated measurement throughout the entire bottom area of the bottle. PA1 Intensity of the laser bundle is modulated in time by a light chopper signal of the photodetector and this is analyzed through a band-pass filter having been tuned to the modulation frequency of the light. This increases sensitivity by reducing the effect of ambient noise-light.
Thus, the known methods do not solve the problem.